1. Field of the Invention
The present invention relates to an inhomogeneity tunable erbium-doped fiber amplifier with a long wavelength gain band and method of blocking the propagation of a backward amplified spontaneous light emission in the same, and more particularly to an inhomogeneity tunable erbium-doped fiber amplifier with a long wavelength gain band and method of blocking the propagation of a backward amplified spontaneous light emission in the amplifier, which is capable of amplifying optical signals of a 1565 to 1605 nm wavelength band transmitted from a transmission line of a WDM (Wavelength Division Multiplexing) fiber transmission system, and tuning backward amplified spontaneous light emissions, thereby improving its output characteristics, such as gain inhomogeneity and a noise figure.
2. Description of the Prior Art
In order to obtain a long wavelength gain band using erbium-doped optical fiber for C-band (Conventional band) whose gain characteristic is optimized at a 1520 to 1560 nm wavelength, 80 m or more of optical fiber should be utilized. However, in this case, there occur problems that the in/out power conversion efficiency of a long wavelength band is decreased to 35% or less, and a noise figure is increased due to low population inversion.
In addition, when an optical amplifier with a long wavelength gain band is designed to amplify multi-wavelength optical signals at a long wavelength band at the same time, the gain spectrum of the erbium-doped fiber cannot be kept the same with regard to the wavelengths of multi-wavelength optical signals, and gain distortion with regard to the wavelengths of optical signals occurs due to the inhomogeneity characteristics of the erbium-doped optical fiber.
In order to overcome the above problems, D. J. DiGiovanni proposed a two-stage optical amplifier with a long wavelength band. This optical amplifier with a long wavelength band, as shown FIG. 1, is comprised of a first stage of a forward stimulation structure using a 980 nm stimulation laser diode, and a second stage of a backward stimulation structure using a 1480 nm stimulation laser diode (see U.S. Pat. No. 5,430,572, filed on Jul. 4, 1995). An optical isolator is positioned between the first stage and the second stage so as to completely block (or isolate) backward amplified spontaneous light emissions that is generated at the second stage and is being propagated to the first stage. The first amplification stage performs amplification using a comparatively short length of an erbium-doped optical fiber, and the second amplification stage performs additional amplification.
An optical amplifier suggested by H. Ono et al. in xe2x80x9cJournal of Lightwave Technology, vol. 17xe2x80x9d, as shown in FIG. 1, has a two-stage amplification arrangement of a hybrid type. The first amplification stage is stimulated using a 980 nm wavelength laser diode, and the second amplification stage is stimulated using a 1480 nm wavelength laser diode. At the first amplification stage, a short length of erbium-doped optical fiber is stimulated by the 980 nm wavelength laser diode so as to decrease a noise figure, while at the second amplification stage, a longer length of erbium-doped optical fiber is stimulated by the 1480 nm wavelength laser diode so as to increase efficiency.
With reference to FIG. 1, a conventional erbium-doped optical fiber amplifier with a long wavelength gain band is described in detail. This conventional amplifier comprises a first stage of a forward stimulation structure using the 980 nm stimulation laser diode 106 for the generation of stimulation light and a second stage of a backward stimulation structure using the 1480 nm stimulation laser diode 108 for the generation of stimulation light. An input optical signal 101 generated at the first stage and stimulation light emitted from the 980 nm laser diode 106 are coupled to each other through a light-isolation type wavelength multiplex optical coupler (IWDM coupler) 105 having a function of isolating light. In order to separate an optical signal output 104 and stimulation light emitted from the 1480 nm laser diode 108 at the second stage and to block backward optical signals being propagated from an output end to the second stage, a wavelength multiplex optical coupler (IWDM coupler) 109 having a function of isolating light is also employed. In this case, the wavelength multiplex optical coupler 109 of the second stage has an insertion loss of 0.3 dB, thereby limiting the increase of a noise figure due to an insertion loss resulting from a front insertion of the wavelength multiplex optical coupler 105 at the first stage.
The optical isolator 107 positioned between the first amplification stage and the second amplification stage blocks a backward amplified spontaneous light emission, which is generated at the second stage and is being propagated to the first stage. That is, the optical isolator 107 allows light being propagated from an output point 102 of the first stage to an input point 103 of the second stage to pass therethrough, and blocks light being propagated from the input point 103 of the second stage to the output point 102 of the first stage. An erbium-doped optical fiber 110 having a cut-off wavelength of 895 nm, which is effective for 980 nm stimulation light, is employed as a gain medium at the first amplification stage. Additionally, an erbium-doped optical fiber 111, which has a cut-off wavelength of 1310 nm and an erbium-doped density of 1000 ppm, is effective for the 1480 nm stimulation light and has high in/output conversion efficiency of 38 to 50% at a long wavelength band, is employed as a gain medium at the second amplification stage.
The 980 nm stimulation light is completely absorbed into the erbium-doped optical fiber 110 at the first amplification stage, so the stimulation light does not exist at the output end 102 of the first stage. The backward amplified spontaneous light emission generated from the second amplification stage should includes a 90% backward amplified spontaneous light emission of a 1560 to 1600 nm band in order that a ratio of 1560 to 1600 band intensity and 1520 to 1560 nm band intensity of the emission is 90:10.
In the optical amplifier with a long wavelength gain band shown in FIG. 1, the spontaneous light emission backwardly propagated from the second amplification stage to the first stage is blocked by the optical isolator 107, so the noise figure and inhomogeneity characteristics of the optical amplifier with a long wavelength gain band are improved and high power output and high efficiency are achieved.
FIGS. 2 to 5 are graphs showing the variations of characteristics in the cases where the backward amplified spontaneous light emission is blocked by the optical isolator in the two-stage optical amplifier with a long wavelength gain band constructed as shown in FIG. 1, and it is not blocked. FIG. 2 is a graph showing in/out power conversion efficiency according to an input light signal. A solid line in FIG. 2 represents the case where the backward-propagating spontaneous light emission is blocked, and a dotted line in FIG. 2 represents the case where the backward-propagating spontaneous light emission is not blocked.
Referring to FIG. 2, when the intensity of an optical signal input is low, the in/out power conversion efficiency is increased in the case where the backward amplified spontaneous emission is blocked; whereas when the intensity of an optical signal input is increased, in/out power conversion efficiency is gradually decreased in the case where the backward amplified spontaneous light emission is blocked, and the efficiency is gradually increased in the case where the backward amplified spontaneous light emission is unblocked. When the backward amplified spontaneous emission is blocked, it has a problem that the in/out power conversion efficiency is lower as the intensity of optical signal input is higher.
FIG. 3 is a graph showing the variation of a noise figure according to an input light signal. A solid line in the FIG. 3 represents the case where the backward amplified spontaneous light emission is blocked, and a dotted line represents the case where the backward amplified spontaneous light emission is not blocked.
Referring to FIG. 3, when the intensity of an optical signal input is low, a noise figure is very low in the case where the backward amplified spontaneous light emission is blocked; whereas when the intensity of optical signal input is increased, a noise figure becomes high. In contrast to the above case, when the intensity of optical signal input is low, a noise figure is very high in the case where the backward amplified spontaneous light emission is unblocked; whereas when the intensity of optical signal input is increased, the noise figure becomes considerably lower. As described above, when the backward amplified spontaneous light emission is blocked, there occurs a problem that a noise figure characteristic is deteriorated as the intensity of input optical signal becomes high.
FIG. 4 is a graph showing the changing characteristics of forward propagating spontaneous emission enhancement of the optical amplifier according to the stimulation light intensity of the 1480 nm stimulation laser diode connected to the second stage. Referring to the FIG. 4, gain balance can be tuned by the backward amplified stimulation light intensity, and an isolation rate or a blocking rate of the backward amplified spontaneous light emission can be also tuned, thereby improving gain and controlling gain imbalance.
FIG. 5 shows the changing characteristics of a gain spectrum in the case where a wavelength of an input optical signal is changed to about 1576 nm and 1596 nm on the basis of a gain spectrum obtained for the input optical signal located near 1586 nm when the backward amplified spontaneous light emission is blocked by the optical isolator 107 in the optical amplifier with a long wavelength gain band shown in FIG. 1. Referring to FIG. 5, it can be understood that a difference of a total gain spectrum according to the wavelength change of the input optical signal is uniform when the backward amplified spontaneous light emission is blocked.
As described above, the two-stage optical amplifier with a long wavelength gain band has an advantageous structure that is capable of improving the in/out power conversion efficiency and a noise figure, and of solving inhomogeneity characteristics. However, it has problems that the in/out power conversion efficiency and a noise figure are deteriorated when the intensity of input optical signal is increased.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an inhomogeneity tunable erbium-doped fiber amplifier with a long wavelength gain band and method of blocking the propagation of a backward amplified spontaneous light emission in the amplifier, in which in two-stage erbium-doped optical fiber amplifier with a long wavelength gain band, the isolation rate of a backward amplified spontaneous light emission generated at a second amplification stage is turned according to intensity of an optical signal inputted into a first amplification stage, thereby improving in/out power conversion efficiency and a noise figure.
In order to accomplish the above object, the present invention provides an inhomogeneity tunable erbium-doped optical fiber amplifier with a long wavelength band, comprising a control device situated between a first amplification stage and a second amplification stage for controlling an isolation rate of a backward amplified spontaneous light emission being propagated from a second amplification stage to the first amplification stage.
Preferably, the control device includes two three-port optical circulators situated between the first amplification stage and the second amplification stage and serially connected to each other, and an optical attenuator for connecting the other ports of the two optical circulators, wherein a forward propagating optical signal is propagated from the first amplification stage through the two optical circulators to the second amplification stage, and the backward amplified spontaneous light emission is propagated from the second amplification stage to the first amplification stage through one of the optical circulators, the optical attenuator and the other optical circulator, so the forward propagating optical signal is propagated without attenuation and the backward amplified spontaneous emission is attenuated by the optical attenuator.
In addition, the present invention provides a method of blocking a backward amplified spontaneous light emission in a two-stage inhomogeneity tunable erbium-doped optical fiber amplifier with a long wavelength gain band, which blocks the backward amplified spontaneous light emission being propagated from a second amplification stage to a first amplification stage, wherein an isolation rate of the backward amplified spontaneous light emission is controlled according to the intensity of an optical signal inputted to the first amplification stage.